First Page:

S R2 R1 IA − at solar noon, it evidently becomes a meridian instrument. It has the advantage also of reflecting the sun when it is just too cloudy for a shadow to be distinct, and in fact you can only see it through darkened glass when the sun is bright. The instrument consists of three small plates of glass put together at their edges in a brass box about 2 inches wide and high, so as to form a hollow prism of any convenient angle, no precision being necessary in this. ABC in this figure is the section of it at right angles to the axis of the prism. The front glass BC is plain; the other two are blackened behind to form reflectors. But though the front glass is transparent, it also reflects, because there are dark ones behind it. SI and IR1 are sections of the planes of incidence and reflection of the sun’s rays from the glass BC, those planes being all parallel to the axis of the prism, which should be approximately parallel to the earth’s axis. Part of the rays passes through that glass, WATER CLOCKS. 11 and is reflected by glass AC to AB, and again reflected there and sent through BC to R2. Now let the angle of incidence, and therefore of first reflection at I, be called A− (A being the angle between the two reflecting glasses), and let the other angles be designated as in the figure. It requires no mathematics beyond the knowledge that the three angles of every triangle = 180, commonly called to see that the angle = − (C A − ) in the small triangle near C; and in the one near A, = − (A ) = C − ; and in the triangle near B, = − (B ) = − (B C − ) = A . That is to say, the angle , made by the plane of emergence of the twice reflected rays with the front glass, differs from that of the once reflected by 2. Therefore, if the prism is so placed as to make = 0, which it will be if the angle of incidence = A, the twice reflected rays will come out parallel to the once reflected, and the two images of the sun will coincide. In fixing the instrument however, we have nothing to do with the angles; but simply to adjust it by trial with a chronometer (for it cannot be done without), so that the images do coincide at solar noon. They were at first made so as to be fixtures on the stone where they were set, and so they were always exposed to the weather, and besides, if cemented in wrongly, they were wrong for ever. To avoid both these evils, I suggested the making of a brass plate to be fixed on the stone, with a raised slip adjustable by screws, against which the instrument is to be laid closely when used, but at other times it may be kept in the house. Some of them are made to turn on an axis parallel to the earth’s axis, and then they can be presented to the sun at other hours besides noon, but only for the given latitude like a sun dial. Some are made adjustable for latitude also. They are moreover made for star observations with the reflectors silvered instead of blackened, on account of the greater feebleness of star light. A table was published to be used with them, showing the time of first and last contact of the two images of the sun for every day in the year, as that observable perhaps more accurately than the time of coincidence; at any rate it gives three chances of observation instead of one. But many people prefer my meridian slit. WATER AND SAND CLOCKS. The earliest time keeping machine is the clepsydra or water clock of the Greeks and Romans, which was no doubt made in various ways. Vitruvius mentions one made as a water wheel, which would probably be very irregular... Continue reading book >>